Generally, a papermaking process includes several steps. An aqueous dispersion of the papermaking fibers is formed into an embryonic web on a formations member, such as Fourdrinier wire, or a twin wire paper machine, where initial dewatering and fiber rearrangement occurs.
In a through-air-drying process, after the initial dewatering, the embryonic web is transported to a through-air-drying belt comprising an air pervious deflection member. The deflection member may comprise a patterned resinous framework having a plurality of deflection conduits through which air may flow under a differential pressure. The resinous framework is joined to and extends outwardly from a woven reinforcing structure. The papermaking fibers in the embryonic web are deflected into the deflection conduits, and water is removed through the deflection conduits to form an intermediate web. The resulting intermediate web is then dried at the final drying stage at which the portion of the web registered with the resinous framework may be subjected to imprinting--to form a multi-region structure.
Through-air drying papermaking belts comprising the reinforcing structure and the resinous framework are described in commonly assigned U.S. Pat. No. 4,514,345 issued to Johnson et al. on Apr. 30, 1985; U.S. Pat. No. 4,528,239 issued to Trokhan on Jul. 9, 1985; U.S. Pat. No. 4,529,480 issued to Trokhan on Jul. 16, 1985; U.S. Pat. No. 4,637,859 issued to Trokhan on Jan. 20, 1987; U.S. Pat. No. 5,334,289 issued to Trokhan et al on Aug. 2, 1994. The foregoing patents are incorporated herein by reference for the purpose of showing preferred constructions of through-air drying papermaking belts. Such belts have been used to produce commercially successful products such as Bounty.RTM. paper towels and Charmin Ultra.RTM. toilet tissue, both produced and sold by the instant assignee.
Presently, the resinous framework of a through-air drying papermaking belt is made by processes which include curing a photosensitive resin with UV radiation according to a desired pattern. Commonly assigned U.S. Pat. No. 5,514,523, issued on May 7, 1996 to Trokhan et al. and incorporated by reference herein, discloses one method of making the papermaking belt using differential light transmission techniques. To make such a belt, a coating of a liquid photosensitive resin is applied to the reinforcing structure. Then, a mask in which opaque regions and transparent regions define a pre-selected pattern is positioned between the coating and a source of radiation, such as UV light. The curing is performed by exposing the coating of the liquid photosensitive resin to the UV radiation from the radiation source through the mask. Typically, the curing radiation comprises both a direct radiation from the source and a reflected radiation from a reflective surface generally having an ellipsoidal and/or parabolic, or other, shape if viewed in a cross-machine directional cross-section. The curing UV radiation passing through the transparent regions of the mask cures (i. e., solidifies) the resin in the exposed areas to form knuckles extending from the reinforcing structure. The unexposed areas, which correspond to the opaque regions of the mask, remain uncured (i. e., fluid) and are subsequently removed.
The angle of incidence of the radiation has an important effect on the presence or absence of taper in the walls of the conduits of the papermaking belt. Radiation having greater parallelism produces less tapered (or more nearly vertical) conduit walls. As the conduits become more vertical, the papermaking belt has a higher air permeability, at a given knuckle area, relative to the papermaking belt having more tapered walls.
Typically, to control the angle of incidence of the curing radiation, the curing radiation may be collimated to permit a better curing of the photosensitive resin in the desired areas, and to obtain a desired angle of taper in the walls of the finished papermaking belt. One means of controlling the angle of incidence of the radiation is a subtractive collimator. The subtractive collimator is, in effect, an angular distribution filter which blocks the UV radiation rays in directions other than those desired. The U.S. Pat. No. 5,514,523 cited above and incorporated herein by reference discloses a method of making the papermaking belt utilizing the subtractive collimator. The common subtractive collimator of the prior art comprises a dark-colored, non-reflective, preferably black, structure comprising series of channels through which the curing radiation may pass in the desired directions. The channels of the prior art's collimator have a comparable size in both the machine direction and the cross-machine direction and are discrete in both the machine direction and the cross-machine direction.
While the subtractive collimator of the prior art helps to orient the radiation rays in the desired directions, the total radiation energy that reaches the photosensitive resin to be cured is reduced because of losses of the radiation energy in the subtractive collimator. Now, it has been found that these losses can be minimized, especially the losses of the curing radiation due to collimation in the machine direction. Since the papermaking belt moves in the machine direction during the manufacturing process, collimating the curing radiation in the machine direction can be achieved by controlling a machine-directional dimension of the aperture through which the curing radiation reaches the photosensitive resin. Furthermore, the ellipsoidal or parabolic general shape of the reflecting surface allows to collimate at least a reflected part of the curing radiation in the machine direction to sufficiently high degree. The collimation of the curing radiation in the cross-machine direction, however, cannot be controlled by adjusting the aperture's cross-machine-directional dimension, simply because the aperture's cross-machine-directional dimension must be no less than the width of the belt being constructed. Also, the ellipsoidal and parabolic reflective surfaces are designed to change the angular distribution of the curing (reflected) radiation primarily in the machine direction, and not the cross-machine direction. Therefore, the curing radiation output and the efficiency of the whole process for making the belt may be significantly increased by reducing losses of the radiation due to collimating the radiation in the machine direction while maintaining the necessary level of collimating in the cross-machine direction.
Therefore, it is an object of the present invention to provide a novel subtractive collimator for use in the processes for curing the photosensitive resin for producing a papermaking belt having the resinous framework, which collimator significantly reduces the loss of the curing energy.
It is another object of the present invention to provide a novel slatted collimator designed to decouple collimation of the curing radiation in the machine direction from the collimation of the curing radiation in the crossmachine direction.
It is also an object of the present invention to provide an improved process for curing a photosensitive resin, using such a slatted collimator of the present invention.